The invention relates to methods of growing plants using a mineral wool growth substrate. In the method, the mineral wool growth substrate comprises a binder, often a hydrophobic binder, to provide structural integrity, and a wetting agent which provides water handling characteristics. The invention also relates to the use of specific types of wetting agent to provide particular characteristics to a mineral wool growth substrate.
It has been well known for many years to grow plants in growth substrates formed from mineral wool. Mineral wool products which are provided as a coherent plug, block, slab or mat generally include a binder, usually an organic binder, in order to provide structural integrity to the product and such binders are commonly hydrophobic once cured. This allows the growth substrate to retain its structure during water irrigation, including use for over one year. However, mineral wool products which are to be used as growth substrates must have a capacity to take up and hold water, which is routinely supplied by an irrigation system to the growth substrate, and re-wetting properties. Accordingly, it has been well known for some years to include a wetting agent in mineral wool products which are to be used as growth substrates.
One early example of this is GB-A-1336426, which describes readily wettable mineral wool products suitable for use as growth substrates. To provide structure and shape, the fibers contain a binder such a phenol formaldehyde resin or urea formaldehyde resin. To provide the required water holding characteristics the product also contains a wetting agent, in an amount of up to 2% by wt. based on the mineral wool. General classes of wetting agents are mentioned, such as anionic and cationic wetting agents, but non-ionic wetting agents are said to be preferred. Examples are alkyl polyglycol ethers such as Triton CF10 and Lissapol N. These are said to be especially valuable where the growth substrate must be subjected to elevated temperatures in order to cure the binder, as they retain adequate wetting properties even after being subjected to temperatures of 200° C. or greater. In cases where the wetting agent is not subjected to elevated temperatures, a wider variety of materials is suggested, including lauryl alcohol. Generally the product is said to be in the form of sheets which can have thickness greater than 1 cm. An example is given of slabs of thickness 5 cm containing cured binder and Triton CF10 wetting agent.
Subsequent publications, and the commercial field itself, have generally followed this preferred approach. For instance, EP-A-889175, DE-A-4208733, WO 92/04824 and DE-A-4024727 all describe systems in which a wetting agent/surfactant is included in the mineral wool growth substrate. In these cases, the nature of the wetting agent is either unspecified or is stated to be a non-ionic material, as in GB-A-1336426. In commercial practice levels of up to 3% non-ionic surfactant, by weight based on the growth substrate, are normally used.
WO93/00797 discloses a growing medium said to be advantageous for improving cultivation control in comparison with sphagnum or peat-based products. According to this publication the growing medium comprises mineral fibers and granulated expanded material secured in a three-dimensional matrix with cured binder. The invention of this document requires that the growing medium comprises at least 30% granulated dried clay and from 0.01 to 10% by weight binder. It is stated that this combination of materials reduces the risk of over-watering of the growing medium and improves water retention.
This document also indicates that it is advantageous to cure the product so that a cohesive mineral fiber matrix is obtained, preventing the sedimentation and/or washing out of the added particulate materials.
It is also stated that anionic surfactant may also be included in the growing medium but no particular advantage is associated with the inclusion of the anionic surfactant.
Whilst the inclusion of binder and wetting agent as described in GB-A-1336426 is generally very helpful, it has certain problems. The first is that the water handling capabilities of the substrate, although good, show room for improvement. Thus, it would be desirable to provide systems which show improved re-saturation properties (ability of the growth substrate to take up water a second time once it has been wetted and then dried), improved water distribution properties (ability of the growth substrate to hold water at a more uniform concentration throughout the height of the growth substrate rather than concentrating at the base), and water retention.
Additionally, one problem which tends to arise with the non-ionic surfactants which have conventionally been used is that they are, during irrigation of the growth substrate, gradually lost from the growth substrate rather than being retained in the mineral wool structure. This means that the wettability properties of the growth substrate gradually decrease over time. This can have an impact on, for instance, the re-saturation properties and also on the water handling properties of a substrate which has been in place for a considerable time, and thus on the performance of the plant and growing process.
Accordingly, there have been other attempts in the art to deal with the problem of providing a mineral wool growth substrate which contains binder material to provide structural integrity but which also has appropriate wettability properties. Examples include the publications EP-A-889175, DE-A-4208733, WO 92/04824 and DE-A-4024727 mentioned above, as well as CA-A-2301248, WO 97/07664 and WO 99/38372, which address the problem by providing specific binders which can provide both structural and wettability properties. CA-A-2301248 discloses a substrate which additionally comprises surfactant.
These latter two approaches, whilst having advantages, in particular the advantage that the wetting agent is not lost during irrigation, necessitate the use of a rather specific and potentially expensive binder material. Further, there is still room for improvement in their water handling properties.
In particular, it is a constant aim of the growth substrate industry to provide a growth substrate which has improved water distribution over its height. It is required to achieve a water to air ratio throughout the growth substrate which is as close as possible to optimum. Naturally, when a growth substrate is irrigated with water, there is a tendency for water concentration at the base to be higher, and often significantly higher, than water concentration in the top region, purely due to the influence of gravity. A number of approaches have been made in order to address this problem, including, for instance, U.S. Pat. No. 5,009,030, which modifies the nature of the substrate itself throughout the height of the block of mineral wool growth substrate, in order to provide greater water retention in the top regions of the block of mineral wool substrate.
This is especially problematic in the context of growth substrates of relatively large height.
GB-A-1336426 describes growth substrates having thickness of 5 cm or greater. Growth substrates can have a wide variety of thicknesses or heights. The particular height chosen is dependent upon the application for which the mineral wool growth substrate is intended.
However, there are some applications in which the substrate must have a particular minimum height. These include the propagation phase, mobile plant systems and transplantation processes, as well as products intended for growing plants in containers. In these applications the known systems present particular problems, because the greater the height of the substrate the greater the problems with retaining sufficient amounts of water in the top regions of the substrate.
Accordingly, it is still, despite the many years during which the industry has used mineral wool materials as growth substrates, an aim to provide materials which exhibit excellent water distribution properties and water retention properties, and which maintain these properties over time, during use, and thus can exhibit excellent re-saturation properties. It is also desirable to achieve this in the context of a product which has good structural integrity and can be made in conventional manner.
There is also the continuing problem of appropriate irrigation methods. In general it is conventional to irrigate mineral wool substrates, especially those of relatively large height, from the top using hoses and drippers. This is necessary to avoid the top of the substrate drying out too much. However, it is generally less convenient to have to provide such systems.
EP-A-1226749 discloses a process for the production of water-absorbing mineral fiber products, which can be used for growing plants. According to this process, during production of the fibers, material is applied to the fibers after formation and before collection, comprising binder, wetting agent and aliphatic polyol. The binder can be a conventional phenol-based resin. The polyol can preferably be ethylene glycol, propylene glycol or trimethylolpropane. The wetting agent can be selected from a long list including salts of higher fatty acids, alkyl or aralkyl sulfates or sulfonates, fatty alcohol sulfates, alkyl phosphates, fatty alcohol ethoxylates, alkyl phenol ethoxylates, fatty amine ethoxylates, fatty acid ethoxylates, alkyl ammonium compounds. Alkyl benzene sulfonate is said to be preferred. No reason is given for this preference.